This invention relates generally to radio frequency circuits and more particularly to radio frequency distributed circuits.
As is known in the art, distributed amplifiers are used to amplify broadband radio frequency signals. In general, a distributed amplifier includes a plurality of transistors, an input transmission line to successively couple the input electrodes of the transistors, to an input terminal, and an output transmission line to successively couple the output electrodes of the transistors to an output terminal. The inherent reactance between the input electrode and the reference electrode of each transistors is taken into consideration when designing the input transmission line. By incorporating this inherent reactance into the design of the transmission line, a broadband network is provided. Similarly, the inherent reactance between the output electrode of each transistor and the reference electrode is also taken into consideration when designing the output transmission line to provide a broadband output network. These arrangements enable distributed amplifiers to have very broadband widths. Additive gain is imparted to an input signal fed to the amplifier since the amplifier operates by having successive portions of the signal coupled to the input electrodes. In response at the output electrodes, amplified portions of the successive signal portions are provided. These output signal portions propagate along the output line and add in phase at the output of the amplifier.
One problem with distributed amplifiers is that the additive gain and the power capabilities of distributed amplifiers are limited and often it is necessary to cascade stages distributed amplifiers to improve gain.
A recent solution to the problem of limited power and gain performance of distributed amplifiers is described in U.S. Pat. No. 4,752,767. In this patent, the matrix amplifier is described. In general, the matrix amplifier or matrix distributed amplifier includes a pair (or more) of distributed amplifiers which have the output electrodes of a first one of the distributed amplifiers sharing a common transmission line with the input electrodes of a second one of the distributed amplifiers. Some advantages are provided with this arrangement. For example, higher gain capabilities are possible from a matrixed set of distributed amplifiers. This is because in addition to additive gain provided to an output signal as in the distributed amplifier, multiplicative gain is also provided to the output signal as a result of successive portions of the signal being coupled through cascaded pairs of distributed amplifiers. Further, since each of artificial transmission lines is lossy, and the matrix requires one less line than a conventional cascade, total gain is increased. These advantages are achieved while reducing the size of the circuit. Since such circuits are often formed as monolithic microwave integrated circuits using semiconductor integration techniques, it is desirable to have a smaller circuit since a smaller circuit inter alia will reduce cost and improve yield over a larger circuit.
One problem, however, exists with matrix amplifiers which limits their usefulness. Since the matrix amplifier includes a pair of distributed amplifiers which share a common line, a difficulty arises in providing D.C. bias to the input electrodes of one of the amplifiers and the output electrodes of the other amplifier along the common line. The bias requirements for the input electrodes of transistors are generally different than the bias requirements for the output electrodes of transistors.
In a paper entitled "Design and Performance of a 2-18 GHZ Monolithic Matrix Amplifier" and in the above-mentioned U.S. Patent, bias is supplied to the output electrodes in each transistor of the plurality of transistors by disposing a resistor in series between the reference potential and the reference electrode. There are several disadvantages with this technique. One disadvantage is that the resistance dissipates power during operation of the matrix amplifier. This reduces the efficiency of the matrix amplifier. Further, this arrangement also provides poor gain control, since gate bias is no longer determined by an external gate bias source but rather is determined by the voltage drop across the resistor connected between the reference electrode and the reference potential. This arrangement, accordingly, does not permit flexible adjustment of the gate bias voltage; and moreover, this arrangement may cause problems if there exist minor differences in the gain characteristics of the transistors employed in matrix amplifier.